MR imaging in carotid artery atherosclerosis plaque characterization

Classifications of atherosclerotic plaque components with T1 and T2* mapping in 11.7 T MRI

BACKGROUND AND AIMS

Histopathology is the gold standard for analysis of atherosclerotic plaques but has drawbacks due to the destructive nature of the method. Ex vivo MRI is a non-destructive method to image whole plaques. Our aim was to use quantitative high field ex vivo MRI to classify plaque components, with histology as gold standard.

METHODS

Surgically resected carotid plaques from 12 patients with recent TIA or stroke were imaged at 11.7 T MRI. Quantitative T1/T2* mapping sequences and qualitative T1/T2* gradient echo sequences with voxel size of 30 × 30 × 60 μm3 were obtained prior to histological preparation, sectioning and staining for lipids, inflammation, hemorrhage, and fibrous tissue. Regions of interest (ROI) were selected based on the histological staining at multiple levels matched between histology and MRI. The MRI parameters of each ROI were then analyzed with quadratic discriminant analysis (QDA) for classification.

RESULTS

A total of 965 ROIs, at 70 levels matched between histology and MRI, were registered based on histological staining. In the nine plaques where three or more plaque components were possible to co-localize with MRI, the mean degree of misclassification by QDA was 16.5 %. One of the plaques contained mostly fibrous tissue and lipids and had no misclassifications, and two plaques mostly contained fibrous tissue. QDA generally showed good classification for fibrous tissue and lipids, whereas plaques with hemorrhage and inflammation had more misclassifications.

CONCLUSION

11.7 T ex vivo high field MRI shows good visual agreement with histology in carotid plaques. T1/T2* maps analyzed with QDA is a promising non-destructive method to classify plaque components, but with a higher degree of misclassifications in plaques with hemorrhage or inflammation.

Magnetic Resonance Imaging Detection of Intraplaque Hemorrhage

Carotid artery atherosclerosis is a major cause of ischemic stroke. For more than 30 years, future stroke risk and carotid stroke etiology have been determined using percent diameter stenosis based on clinical trials in the 1990s. In the past 10 years, magnetic resonance imaging (MRI) sequences have been developed to detect carotid intraplaque hemorrhage. By detecting carotid intraplaque hemorrhage, MRI identifies potential stroke sources that are often overlooked by lumen imaging. In addition, MRI can dramatically improve assessment of future stroke risk beyond lumen stenosis alone. In this review, we discuss the use of heavily T1-weighted MRI sequences used to detect carotid intraplaque hemorrhage. In addition, advances in ciné imaging, motion robust techniques, and specialized neck coils will be reviewed. Finally, the clinical use and future impact of MRI plaque hemorrhage imaging will be discussed.

T2-prepared segmented 3D-gradient-echo for fast T2-weighted high-resolution three-dimensional imaging of the carotid artery wall at 3T: a feasibility study

BACKGROUND

The multi-contrast assessment of the carotid artery wall has become an important diagnostic tool for the characterization of atherosclerotic plaque and vessel wall thickening. For providing the required T1-, T2-, and proton density weighted contrast, multi-slice turbo spin echo (TSE) techniques are normally applied. The straightforward extension of the TSE techniques to volumetric imaging of large sections of the carotid arteries is limited by the resulting long acquisition times. Where the acquisition of a T1-weighted contrast can be accelerated by applying a T1-weighted fast gradient echo technique, acceleration of the T2-weighted contrast is not as straightforward.

METHODS

In this work, the combination of a T2 preparation and a conventional fast gradient echo technique (T2P-3DGE) was evaluated for rapid acquisition of a T2-weighted image contrast. Acquisition parameters were optimized in an initial in vitro study in direct comparison to the conventional T2-weighted TSE (T2W-3DTSE) technique. Subsequently, the T2P-3DGE technique was evaluated in vivo.

RESULTS

In direct comparison, the T2P-3DGE sequence provided similar T2 contrast as the respective T2W-3DTSE sequence. After correction of an observed intensity offset, most likely caused by the additional T1-weighting of the T2P-3DGE sequence, no significant difference between the two T2-weighted sequences were observed in phantom data. The good correlation of the image contrast between the two sequences was confirmed in the initial in-vivo study, proving a potential reduction of the scan time for T2P-3DGE to 25% of the respective T2W-3DTSE technique.

CONCLUSION

The in vitro as well as the in vivo results clearly indicate the potential of the T2P-3DGE technique for providing similar T2 image contrast as in the conventional techniques. Thereby, the acquisition times could be substantially reduced to about 25% of the respective 3D-TSE technique.

Correlation of Carotid Intraplaque Hemorrhage and Stroke Using 1.5 T and 3 T MRI

Carotid therosclerotic disease causes approximately 25% of the nearly 690,000 ischemic strokes each year in the United States. Current risk stratification based on percent stenosis does not provide specific information on the actual risk of stroke for most individuals. Prospective randomized studies have found only 10 to 12% of asymptomatic patients will have a symptomatic stroke within 5 years. Measurements of percent stenosis do not determine plaque stability or composition. Reports have concluded that cerebral ischemic events associated with carotid plaque are intimately associated with plaque instability. Analysis of retrospective studies has found that plaque composition is important in risk stratification. Only MRI has the ability to identify and measure the detailed components and morphology of carotid plaque and provides more detailed information than other currently available techniques. MRI can accurately detect carotid hemorrhage, and MRI identified carotid hemorrhage correlates with acute stroke.

Ex vivo study of carotid endarterectomy specimens: quantitative relaxation times within atherosclerotic plaque tissues

PURPOSE

Previous studies reporting relaxation times within atherosclerotic plaque have typically used dedicated small-bore high-field systems and small sample sizes. This study reports quantitative T(1), T(2) and T(2) relaxation times within plaque tissue at 1.5 T using spatially co-matched histology to determine tissue constituents.

METHODS

Ten carotid endarterectomy specimens were removed from patients with advanced atherosclerosis. Imaging was performed on a 1.5-T whole-body scanner using a custom built 10-mm diameter receive-only solenoid coil. A protocol was defined to allow subsequent computation of T(1), T(2) and T(2) relaxation times using multi-flip angle spoiled gradient echo, multi-echo fast spin echo and multi-echo gradient echo sequences, respectively. The specimens were subsequently processed for histology and individually sectioned into 2-mm blocks to allow subsequent co-registration. Each imaging sequence was imported into in-house software and displayed alongside the digitized histology sections. Regions of interest were defined to demarcate fibrous cap, connective tissue and lipid/necrotic core at matched slice-locations. Relaxation times were calculated using Levenberg-Marquardt's least squares curve fitting algorithm. A linear-mixed effect model was applied to account for multiple measurements from the same patient and establish if there was a statistically significant difference between the plaque tissue constituents.

RESULTS

T(2) and T(2) relaxation times were statistically different between all plaque tissues (P=.026 and P=.002 respectively) [T(2): lipid/necrotic core was lower 47 ± 13.7 ms than connective tissue (67 ± 22.5 ms) and fibrous cap (60 ± 13.2 ms); T(2): fibrous cap was higher (48 ± 15.5 ms) than connective tissue (19 ± 10.6 ms) and lipid/necrotic core (24 ± 8.2 ms)]. T(1) relaxation times were not significantly different (P=.287) [T(1): Fibrous cap: 933 ± 271.9 ms; connective tissue (1002 ± 272.9 ms) and lipid/necrotic core (1044 ± 304.0 ms)]. We were unable to demarcate hemorrhage and calcium following histology processing.

CONCLUSIONS

This study demonstrates that there is a significant difference between qT(2) and qT(2) in plaque tissues types. Derivation of quantitative relaxation times shows promise for determining plaque tissue constituents.

Multi-contrast high spatial resolution black blood inner volume three-dimensional fast spin echo MR imaging in peripheral vein bypass grafts

The purpose of this study is to primarily evaluate the lumen area and secondarily evaluate wall area measurements of in vivo lower extremity peripheral vein bypass grafts patients using high spatial resolution, limited field of view, cardiac gated, black blood inner volume three-dimensional fast spin echo MRI. Fifteen LE-PVBG patients prospectively underwent ultrasound followed by T1-weighted and T2-weighted magnetic resonance (MR) imaging. Lumen and vessel wall areas were measured by direct planimetry. For graft lumen areas, T1- and T2-weighted measurements were compared with ultrasound. For vessel wall areas, differences between T1- and T2-weighted measurements were evaluated. There was no significant difference between ultrasound and MR lumen measurements, reflecting minimal MR blood suppression artifact. Graft wall area measured from T1-weighted images was significantly larger than that measured from T2-weighted images (P < 0.001). The mean of the ratio of T1- versus T2-weighted vessel wall areas was 1.59 (95% CI: 1.48-1.69). The larger wall area measured on T1-weighted images was due to a significantly larger outer vessel wall boundary. Very high spatial resolution LE-PVBG vessel wall MR imaging can be performed in vivo, enabling accurate measurements of lumen and vessel wall areas and discerning differences in those measures between different tissue contrast weightings. Vessel wall area differences suggest that LE-PVBG vessel wall tissues produce distinct signal characteristics under T1 and T2 MR contrast weightings.

Preliminary studies on human aldosterone synthase (CYP11B2) gene polymorphism, matrix metalloprotease-9, apoptosis, and carotid atherosclerosis plaque size by proton magnetic resonance imaging

Hypothesis. Aldosterone has direct or indirect effects on atherosclerosis, and polymorphisms occur in the gene encoding aldosterone synthase (CYP11B2), the enzyme catalysing aldosterone biosynthesis. Genetic variations in aldosterone synthesis may influence progression of carotid atherosclerosis.

Materials and methods. Ten subjects were genotyped through the use of the polymerase chain reaction for two diallelic polymorphisms in CYP11B2: one in the transcriptional regulatory region (promotor) and the other in the second intron. In vivo plaque size was estimated by H-1 magnetic resonance imaging using gradient echo pulse sequence. Media-intima thickness and ex vivo plaque in endarterectomy samples were measured by histology. Matrix metalloprotease (MMP)-9 was stained in endarterectomy histology sections and apoptosis index was counted in these sections.

Results. The CYP11B2 promoter genotype patterns were associated significantly with the plaque size in carotid artery (r2=0.9987; p=0.001), MMP-9 levels (r 2=0.9878; p=0.0001) and apoptotic indices (r2=0.9495; p=0.005) by multiple regression analysis. The media-intima thickness was not significantly correlated with genotype patterns.

Conclusion. Genetic variations in aldosterone synthase (CYP11B2) gene are associated with the progression of atherosclerotic plaque size, MMP-9 and apoptosis in the carotid artery.

In vivo differentiation of two vessel wall layers in lower extremity peripheral vein bypass grafts: application of high-resolution inner-volume black blood 3D FSE

Lower extremity peripheral vein bypass grafts (LE-PVBG) imaged with high-resolution black blood three-dimensional (3D) inner-volume (IV) fast spin echo (FSE) MRI at 1.5 Tesla possess a two-layer appearance in T1W images while only the inner layer appears visible in the corresponding T2W images. This study quantifies this difference in six patients imaged 6 months after implantation, and attributes the difference to the T(2) relaxation rates of vessel wall tissues measured ex vivo in two specimens with histologic correlation. The visual observation of two LE-PVBG vessel wall components imaged in vivo is confirmed to be significant (P < 0.0001), with a mean vessel wall area difference of 6.8 +/- 2.7 mm(2) between contrasts, and a ratio of T1W to T2W vessel wall area of 1.67 +/- 0.28. The difference is attributed to a significantly (P < 0.0001) shorter T(2) relaxation in the adventitia (T(2) = 52.6 +/- 3.5 ms) compared with the neointima/media (T(2) = 174.7 +/- 12.1 ms). Notably, adventitial tissue exhibits biexponential T(2) signal decay (P < 0.0001 vs monoexponential). Our results suggest that high-resolution black blood 3D IV-FSE can be useful for studying the biology of bypass graft wall maturation and pathophysiology in vivo, by enabling independent visualization of the relative remodeling of the neointima/media and adventitia.